The car averaged 19.8 MPG on the way to Vegas from Los Angeles which had about 45 minutes of stop and go traffic on Fwy 10 and 15, not bad at all for my '08 ACR w/ 3.55 gears. The best part was that I didn't have to stop mid way for a gas stop. But coming back from Vegas w/ less traffic and traveling toward the coast (more downhill) averaged only 17 MPG and had to stop for gas 30 miles from home! So could it be due to being pedal to the metal for a day at the track got the ECU thinking it needed to dump more fuel into the engine and dropped the gas mileage by 15%?